Dynamics of human respiratory virus-specific CD8+ T cell responses in blood and airways during episodes of common cold

We determined the dynamics of CD8(+) T cells specific for influenza virus and respiratory syncytial virus in blood and tracheostoma aspirates of children during the course of respiratory infections. We showed that during localized respiratory infections the ratio of activated effector CD8(+) T cells to resting memory/naive CD8(+) T cells in peripheral blood increased significantly. Furthermore, the number of effector/memory T cells specific for respiratory viruses declined in blood and increased in the airways, suggesting that these T cells redistributed from blood to airways. T cells specific for the infecting virus were present in the airways for longer periods at increased levels than nonspecifically recruited bystander T cells. After clearance of the infection, the ratio of resting memory and naive CD8(+) T cells normalized in peripheral blood and also memory T cell numbers specific for unrelated viruses that declined during the infection due to bystander recruitment were restored. Taken together, these results showed a significant systemic T cell response during relatively mild secondary infections and extensive dynamics of virus-specific and nonspecific Ag-experienced T cells.     

Nonspecific recruitment of memory CD8+ T cells to the lung airways during respiratory virus infections

Previous studies have shown that heterologous viral infections have a significant impact on pre-existing memory T cell populations in secondary lymphoid organs through a combination of cross-reactive and bystander effects. However, the impact of heterologous viral infections on effector/memory T cells in peripheral sites is not well understood. In this study, we have analyzed the impact of a heterologous influenza virus infection on Sendai virus-specific CD8(+) effector/memory cells present in the lung airways. The data show a transient increase in the numbers of Sendai virus nucleoprotein 324-332/K(b)-specific CD8(+) memory T cells in the airways of the influenza-infected mice peaking around day 4 postinfection. Intratracheal transfer studies and 5-bromo-2'-deoxyuridine incorporation demonstrate that this increase is due to the recruitment of resting memory cells into the airways. In addition, the data show that these immigrating memory cells are phenotypically distinct from the resident memory T cells of the lung airways. A similar influx of nonproliferating Sendai virus nucleoprotein 324-332/K(b)-specific CD8(+) memory T cells is also induced by a secondary (homologous) infection with Sendai virus. Together, these data suggest that inflammation can accelerate memory T cell migration to nonlymphoid tissues and is a part of the normal recall response during respiratory infections.     

Independent regulation of lymphocytic choriomeningitis virus-specific T cell memory pools: relative stability of CD4 memory under conditions of CD8 memory T cell loss

Infection of mice with a series of heterologous viruses causes a reduction of memory CD8(+) T cells specific to viruses from earlier infections, but the fate of the virus-specific memory CD4(+) T cell pool following multiple virus infections has been unknown. We have previously reported that the virus-specific CD4(+) Th precursor (Thp) frequency remains stable into long-term immunity following lymphocytic choriomeningitis virus (LCMV) infection. In this study, we questioned whether heterologous virus infections or injection with soluble protein CD4 Ags would impact this stable LCMV-specific CD4(+) Thp memory pool. Limiting dilution analyses for IL-2-producing cells and intracellular cytokine staining for IFN-gamma revealed that the LCMV-specific CD4(+) Thp frequency remains relatively stable following multiple heterologous virus infections or protein Ag immunizations, even under conditions that dramatically reduce the LCMV-specific CD8(+) CTL precursor frequency. These data indicate that the CD4(+) and CD8(+) memory T cell pools are regulated independently and that the loss in CD8(+) T cell memory following heterologous virus infections is not a consequence of a parallel loss in the memory CD4(+) T cell population.     

Variable patterns of programmed death-1 expression on fully functional memory T cells after spontaneous resolution of hepatitis C virus infection

The inhibitory receptor programmed death-1 (PD-1) is present on CD8(+) T cells in chronic hepatitis C virus (HCV), but expression patterns in spontaneously resolving infections are incompletely characterized. Here we report that PD-1 was usually absent on memory CD8(+) T cells from chimpanzees with resolved infections, but sustained low-level expression was sometimes observed in the absence of apparent virus replication. PD-1-positive memory T cells expanded and displayed antiviral activity upon reinfection with HCV, indicating conserved function. This animal model should facilitate studies of whether PD-1 differentially influences effector and memory T-cell function in resolved versus persistent human infections.     

Renewal of peripheral CD8+ memory T cells during secondary viral infection of antibody-sufficient mice

Kinetic studies and short pulses of injected 5-bromo-2-deoxyuridine have been used to analyze the development and renewal of peripheral CD8(+) memory T cells in the lungs during primary and secondary respiratory virus infections. We show that developing peripheral CD8(+) memory T cells proliferate during acute viral infection with kinetics that are indistinguishable from those of lymphoid CD8(+) memory T cells. Secondary exposure to the same virus induces a new round of T cell proliferation and extensive renewal of the peripheral and lymphoid CD8(+) memory T cell pools in both B cell-deficient mice and mice with immune Abs. In mice with virus-specific Abs, CD8(+) T cell proliferation takes place with minimal inflammation or effector cell recruitment to the lungs. The delayed arrival of CD8(+) memory T cells to the lungs of these animals suggests that developing memory cells do not require the same inflammatory signals as effector cells to reach the lung airways. These studies provide important new insight into mechanisms that control the maintenance and renewal of peripheral memory T cell populations during natural infections.     

Bystander sensitization to activation-induced cell death as a mechanism of virus-induced immune suppression

Viral infections which induce strong T-cell responses are often characterized by a period of transient immunodeficiency associated with the failure of host T cells to proliferate in response to mitogens or to mount memory recall responses to other antigens. During acute infections, most of the activated, proliferating virus-specific T cells are sensitized to undergo apoptosis on strong T-cell receptor (TCR) stimulation, but it has not been known why memory T cells not specific for the virus fail to proliferate on exposure to their cognate antigen. Using a lymphocytic choriomeningitis virus (LCMV) infection model in which LCMV-immune Thy 1.1(+) splenocytes are adoptively transferred into Thy 1.2(+) LCMV carrier mice, we demonstrate here that T cells clearly defined as not specific for the virus are sensitized to undergo activation-induced cell death on TCR stimulation in vitro. This bystander sensitization was in part dependent on the expression of Fas ligand (FasL) on the activated virus-specific cells and gamma interferon (IFN-gamma) receptor expression on the bystander T cells. We propose that FasL from highly activated antiviral T cells may sensitize IFN-gamma-conditioned T cells not specific for the virus to undergo apoptosis rather than to proliferate on encountering antigen. This may in part explain the failure of memory T cells to respond to recall antigens during acute and persistent viral infections.     

Virtual memory cells make a major contribution to the response of aged influenza-naïve mice to influenza virus infection

Background

A diverse repertoire of naïve T cells is thought to be essential for a robust response to new infections. However, a key aspect of aging of the T cell compartment is a decline in numbers and diversity of peripheral naïve T cells. We have hypothesized that the age-related decline in naïve T cells forces the immune system to respond to new infections using cross-reactive memory T cells generated to previous infections that dominate the aged peripheral T cell repertoire.

Results

Here we confirm that the CD8 T cell response of aged, influenza-naïve mice to primary infection with influenza virus is dominated by T cells that derive from the memory T cell pool. These cells exhibit the phenotypic characteristics of virtual memory cells rather than true memory cells. Furthermore, we find that the repertoire of responding CD8 T cells is constrained compared with that of young mice, and differs significantly between individual aged mice. After infection, these virtual memory CD8 T cells effectively develop into granzyme-producing effector cells, and clear virus with kinetics comparable to naïve CD8 T cells from young mice.

Conclusions

The response of aged, influenza-naive mice to a new influenza infection is mediated largely by memory CD8 T cells. However, unexpectedly, they have the phenotype of VM cells. In response to de novo influenza virus infection, the VM cells develop into granzyme-producing effector cells and clear virus with comparable kinetics to young CD8 T cells.

     

Activated antigen-specific CD8+ T cells persist in the lungs following recovery from respiratory virus infections

The poor correlation between cellular immunity to respiratory virus infections and the numbers of memory CD8(+) T cells in the secondary lymphoid organs suggests that there may be additional reservoirs of T cell memory to this class of infection. Here we identify a substantial population of Ag-specific T cells in the lung that persist for several months after recovery from an influenza or Sendai virus infection. These cells are present in high numbers in both the airways and lung parenchyma and can be distinguished from memory cell populations in the spleen and peripheral lymph nodes in terms of the relative frequencies among CD8(+) T cells, activation status, and kinetics of persistence. In addition, these cells are functional in terms of their ability to proliferate, express cytolytic activity, and secrete cytokines, although they do not express constitutive cytolytic activity. Adoptive transfer experiments demonstrated that the long-term establishment of activated T cells in the lung did not require infection in the lung by a pathogen carrying the inducing Ag. The kinetics of persistence of Ag-specific CD8(+) T cells in the lung suggests that they play a key role in protective cellular immunity to respiratory virus infections.     

Cutting edge: effector memory CD8+ T cells play a prominent role in recall responses to secondary viral infection in the lung

The relative contributions of CD62L(high) (central) memory and CD62L(low) (effector) memory T cell populations to recall responses are poorly understood, especially in the respiratory tract. In this study, we took advantage of a dual-adoptive transfer system in the mouse to simultaneously follow the recall response of effector and central memory subpopulations to intranasal parainfluenza virus infection. Using MHC class I and class II multimers, we tracked the responses of Ag-specific CD8(+) and CD4(+) memory T cells in the same animals. The data show that effector memory T cells mounted recall responses that were equal to, or greater than, those mounted by central memory T cells. Moreover, effector memory T cells were more efficient at subsequently establishing a second generation of memory T cells. These data contrast with other studies indicating that central memory CD8(+) T cells are the prominent contributors to systemic virus infections.     

Recall proliferation potential of memory CD8+ T cells and antiviral protection

Memory CD8+ T cells play a crucial role in mediating protection from infection with viruses and other intracellular pathogens. Memory T cells are not a homogenous cellular population and may be separated into central memory T cells with substantial recall proliferation capacity and effector memory T cells with limited recall proliferation capacity. It has been suggested that the protective capacity of effector memory T cells is more limited than that of central memory T cells in viral infections. Here, we show that pronounced recall proliferation potential is indeed key for protection against lymphocytic choriomeningitis virus, which replicates in central lymphoid organs and is controlled by contact-dependent lysis of infected cells. In contrast, recall proliferation competence is not sufficient for protection against vaccinia virus, which is replicating in peripheral solid organs and is controlled by cytokines. To protect against vaccinia virus, high numbers of effector-like T cells were required to be present in peripheral tissue before viral challenge. These data indicate that the protective capacity of different subpopulations of memory T cells may vary dependent on the nature and the route of the challenge infection, which must be considered in T cell-based vaccine design.     

The dynamics of the cellular immune response to HIV infection: implications for vaccination

Recent advances in measuring T-cell responses to viruses have led to new insights into how these T cells respond. In the acute infection there are massive CD8+ T-cell responses to both Epstein-Barr virus (EBV) and to human immunodeficiency virus (HIV). Many of these T cells are effector cells and only a minority appear to be capable of maintaining immunological memory. In persistent virus infections, high levels of antigen-specific effector cells persist. If virus does not persist, the effectors fade in number but memory is maintained and is primed to react rapidly to a new challenge. A vaccine that stimulates only T-cell responses may protect when these memory cells respond rapidly enough to generate high numbers of effectors before the infecting virus becomes established.     

流感病毒交叉反应性记忆T细胞的研究进展

流感病毒是长期威胁人类健康最主要的病毒之一.灭活流感疫苗主要产生针对病毒血凝素的菌株特异性抗体,当新出现的流感病毒株与疫苗株不匹配时,疫苗的有效性将会大大降低.由于季节性流感病毒的抗原漂移和突变的持续出现,人们迫切地需要找到更广泛的保护方式.在以往的研究中,人们逐渐意识到细胞免疫的重要性,尤其是预先存在的记忆T细胞能靶...     

Memory T cell populations in the lung airways are maintained by continual recruitment

Effector memory T cell populations in the periphery play a key role in cellular immune responses to secondary infections. However, it is unclear how these populations are maintained under steady-state conditions in nonlymphoid peripheral sites, such as the lung airways. In this study, we show that LFA-1 expression is selectively down-regulated following entry of memory T cells into the lung airways. Using Sendai virus as a mouse model of respiratory virus infection, we use LFA-1 expression levels to demonstrate that effector memory T cell populations in the lung airways are maintained by continual recruitment of new cells from the circulation. The rate of memory cell recruitment is surprisingly rapid, resulting in replacement of 90% of the population every 10 days, and is maintained for well over 1 year following viral clearance. These data indicate that peripheral T cell memory is dynamic and depends on a systemic source of T cells.     

A model of human immunodeficiency virus infection in T helper cell clones

We present a mathematical model of the activation and proliferation of a clone of T helper cells in response to a replicating antigen. This is able to show types of behaviour akin to persistent infection and to immune memory. This model is expanded to include the infection and destruction of activated T helper cells by human immunodeficiency virus and the growth of a population of circulating human immunodeficiency virus. The resulting model is used to investigate the circumstances under which the human immunodeficiency virus can destabilize persistent infections and destroy immune memory, and to illustrate the impact of antigenic stimulation of infected T helper cell clones upon human immunodeficiency virus replication rates.     

Cutting edge: virus-specific CD4+ memory T cells in nonlymphoid tissues express a highly activated phenotype

Recent studies have shown that CD4(+) memory T cells persist in nonlymphoid organs following infections. However, the development and phenotype of these peripheral memory cells are poorly defined. In this study, multimerized MHC-Ig fusion proteins, with a covalently attached peptide sequence from the Sendai virus hemagglutinin/neuraminidase gene, have been used to identify virus-specific CD4(+) T cells during Sendai virus infection and the establishment of peripheral CD4(+) memory populations in the lungs. We show declining frequencies of virus-specific CD4(+) T cells in the lungs over the course of approximately 3 mo after infection. Like peripheral CD8(+) T cells, the CD4(+) have an acutely activated phenotype, suggesting that a high level of differentiation is required to reach the airways and persist as memory cells. Differences in CD25 and CD11a expression indicate that the CD4(+) cells from the lung airways and parenchyma are distinct memory populations.     

Induction of telomerase activity and maintenance of telomere length in virus-specific effector and memory CD8+ T cells

Acute viral infections induce extensive proliferation and differentiation of virus-specific CD8+ T cells. One mechanism reported to regulate the proliferative capacity of activated lymphocytes is mediated by the effect of telomerase in maintaining the length of telomeres in proliferating cells. We examined the regulation of telomerase activity and telomere length in naive CD8+ T cells and in virus-specific CD8+ T cells isolated from mice infected with lymphocytic choriomeningitis virus. These studies reveal that, compared with naive CD8+ T cells, which express little or no telomerase activity, Ag-specific effector and long-lived memory CD8+ T cells express high levels of telomerase activity. Despite the extensive clonal expansion that occurs during acute lymphocytic choriomeningitis virus infection, telomere length is maintained in both effector and memory CD8+ T cells. These results suggest that induction of telomerase activity in Ag-specific effector and memory CD8+ T cells is important for the extensive clonal expansion of both primary and secondary effector cells and for the maintenance and longevity of the memory CD8+ T cell population.     

Comprehensive early and lasting loss of memory CD8 T cells and functional memory during acute and persistent viral infections

Viral infections have been shown to induce lymphopenias that lower memory CD8 T cell frequencies, and they also have been shown to cause a permanent loss of memory cells specific to previously encountered pathogens. In this study, the patterns and significance of virus-induced memory CD8 T cell depletion were examined in mice immune to heterologous (Pichinde, vesicular stomatitis, vaccinia) viruses and subsequently challenged with acute or persistent lymphocytic choriomeningitis virus infections. Memory CD8 T cell loss was comprehensive and occurred in both lymphoid and peripheral tissues of the immune host. The impact of the loss of memory T cells was reflected by in vivo cytotoxicity assays, which showed decreased clearance of epitope-expressing targets. Memory CD8 T cell loss occurred very early (day 2) after infection, and was thereafter sustained, consistent more with an active deletion model than with a competition model. Cross-reactive T cells, in contrast, increased in number, but memory cells were reduced whether or not there was competition from cross-reactive T cells. Memory T cell loss was more profound during persistent infection than after acute infection. Adoptive transfer studies showed that, unlike the resolved acute infection, in which the reduced memory frequencies became stable, memory T cell loss was a continuously ongoing process during persistent infection. This study therefore links an early virus-induced lymphopenia to a subsequent long-term loss of CD8 T cell memory and offers a new mechanism for immune deficiency during persistent viral infections.     

Expression of the interleukin-7 receptor alpha chain (CD127) on virus-specific CD8+ T cells identifies functionally and phenotypically defined memory T cells during acute resolving hepatitis B virus infection

Virus-specific CD8+ T cells play a central role in the outcome of several viral infections, including hepatitis B virus (HBV) infection. A key feature of virus-specific CD8+ T cells is the development of memory. The mechanisms resulting in the establishment of T-cell memory are still only poorly understood. It has been suggested that T-cell memory may depend on the survival of virus-specific CD8+ T cells in the contraction phase. Indeed, a population of effector cells that express high levels of the interleukin-7 receptor alpha chain (CD127) as the precursors of memory CD8+ T cells has recently been identified in mice. However, very little information is currently available about the kinetics of CD127 expression in an acute resolving viral infection in humans and its association with disease pathogenesis, viral load, and functional and phenotypical T-cell characteristics. To address these important issues, we analyzed the HBV-specific CD8+ T-cell response longitudinally in a cohort of six patients with acute HBV infection who spontaneously cleared the virus. We observed the emergence of CD127 expression on antigen-specific CD8+ memory T cells during the course of infection. Importantly, the up-regulation of CD127 correlated phenotypically with a loss of CD38 and PD-1 expression and acquisition of CCR7 expression: functionally with an enhanced proliferative capacity and clinically with the decline in serum alanine aminotransferase levels and viral clearance. These results suggest that the expression of CD127 is a marker for the development of functionally and phenotypically defined antigen-specific CD8+ memory T cells in cleared human viral infections.